1. Field of the Invention
The present invention relates to a portable radio terminal, an antenna selector control method for use in the portable radio terminal, and an antenna selector control program. In particular, the present invention relates to a portable radio terminal in which a transmission signal is sent to an antenna through an analog switch circuit, an analog switch circuit control method for use in the portable radio terminal, and an analog switch circuit control program.
2. Description of the Related Art
Portable radio terminals include mobile phones conforming to PDC (Personal Digital Cellular), GSM (Global System for Mobile Communications), CDMA (Code Division Multiple Access), PHS (Personal Handy-phone System), etc., PDAs (Personal Digital Assistants) having a radio communication function, notebook personal computers having a radio communication function, and the like. With a portable radio terminal having only a single antenna, multipaths may be occurred depending on the environment where it is used, due to reflection, diffraction, or scattering of a radio wave caused by objects like geographical features and buildings. This may deteriorate the condition of signal transmission/reception of the portable radio terminal. Also, the positional relation between the antenna and the human body varies between when the user holds the portable radio terminal close to the head to have a talk and when the user removes the portable radio terminal from his or her head to see its display, resulting in variation of the radio wave transmission/reception condition. Even when the radio wave transmission/reception condition is good in one situation, the condition may deteriorate in other situations. The signal transmission/reception condition of the portable radio terminal may thus deteriorate. Also, with a folding-type portable radio terminal, the positional relation between the terminal body and the antenna varies when the terminal is folded or opened, which also influenced the radio wave transmission/reception condition. Even when the radio wave transmission/reception condition is good in one situation, the condition may deteriorate in the other situation. The signal transmission/reception condition of the portable radio terminal may thus deteriorate.
For the reasons above, recent portable radio terminals are often provided with two or more antennas. Such a portable radio terminal selects and uses one of the antennas that is offering a good transmission/reception condition. Such a portable radio terminal has two or more transmission signal paths from the radio section to these antennas, and switches the paths using an antenna switch.
A portable radio terminal having such an antenna switch requires that the signal be inputted in such a range that the IMD characteristic of the analog switch circuit does not deteriorate. This imposes restrictions on design.
JP Laid-Open patent No. 5-175877A discloses a time division multiplexing transceiver circuit that switches impedance between transmission and reception. In a high-frequency signal switching device described in the prior art, a voltage from a voltage converter is applied to both ends of a variable-capacitance diode through transmission lines. The voltage converter is supplied with power from a power supply and the output voltage from the voltage converter is controlled by a CPU (Central Processing Unit). The impedance is controlled by the voltage value applied to the variable-capacitance diode.
JP Laid-open patent No. 9-325059A discloses an ultrasonic current meter for measuring the velocity of a fluid. A power-supply control circuit of the described ultrasonic current meter causes a voltage converter circuit to operate to accumulate power in a capacitive element until an ultrasonic wave transmission start signal is inputted, and the power-supply control circuit halts the operation of the voltage converter circuit when the transmission start signal is inputted from a controller. Thus, in this ultrasonic current meter, the power-supply control circuit halts the operation of the voltage converter circuit when a reception circuit makes a reception judgment, and so no noise nor ripple is generated by the voltage converter circuit when the reception timing is determined. This reduces measuring errors and enhances precision.